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Chapter 12
Muscles
Part 3
For Monday, start with slide # 26
Exam 3 will be on Monday November 21
Will cover chapters 11, 12, 13
May cover more (depends on how far we get)
Figure 12-7
Copyright © 2010 Pearson Education, Inc.
Excitation-Contraction (E- C) Coupling
1. Acetylcholine (ACh) is released from the somatic
motor neuron
2. ACh initiates an action potential in the muscle fiber
3. The muscle AP triggers calcium ion release from the
sarcoplasmic reticulum
4. Calcium ion combines with troponin and initiates
contraction
Figure 12-11a, part 1
Copyright © 2010 Pearson Education, Inc.
1. Acetylcholine (ACh) is released from the somatic
motor neuron
– ACh binds to the ACh receptor-channels on the
motor end plate of the muscle fiber
– When the channels open, Na+ and K+ enter the
channels. Na+ influx exceeds K+ efflux
– Na+ influx depolarizes the membrane, creating an
end-plate potential (EPP)
– EPPs always reach threshold and always initiate
an AP
Figure 12-11a, overview
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2. ACh initiates an action potential in the muscle fiber
– The AP is conducted across the surface of the
muscle fiber and into the T-tubules
– This is accomplished by the sequential opening of
voltage-gated Na+ channels
– Similar to the AP conduction in an axon, but a lot
slower in skeletal muscle
3. The muscle AP triggers calcium ion release from the
sarcoplasmic reticulum
– Free cystolic Ca++ levels in a resting muscle are
normally quite low
– After an AP, Ca++ levels increase 100 fold
– When cystolic Ca++ levels are high, Ca++ binds to
troponin, tropomyosin moves to the “on” position,
and contraction occurs
Figure 12-11b, overview
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Molecular Events
Transduction of the electrical signal into a calcium
signal requires 2 different membrane proteins
– DHP receptor
– RyR receptor
Molecular Events
DHP (Dihydropyridine) Receptor
– Found in T-tubule membrane
– It is a voltage-sensing L-type calcium channel
– The DHP receptors do not form open channels
– Rather, they are mechanically linked to Ca++
release channels in the sarcoplasmic reticulum
RyR (ryanodine) receptors
– These are the Ca++ release channels
Molecular Events
When the depolarization of the AP reaches the DHP
receptor, the receptor changes conformation (shape)
This change opens the RyR Ca++ release channels in
the sarcoplasmic reticulum
Stored Ca++ ions then flow down their electrochemical
gradient into the cytosol where the Ca++ ions initiate
contraction
Figure 12-11b, overview
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Molecular Events
To end the contraction, the sarcoplasmic reticulum
pumps Ca++ ions back into its lumen, using a Ca++ATPase
As the free Ca++ ion concentration decreases, Ca++
releases from troponin, tropomyosin slides back into
place and blocks actin's myosin-binding site
As crossbridges release, the muscle fiber relaxes
Molecular Events
The discovery that Ca++ and not the AP is the signal
for contraction led to lots of research on Ca++
Ca++ was once thought to only be a signalling
molecule in muscle cells, but now has been found to
be an almost universally used second messenger
molecule
Timing of events in E-C coupling (Fig. 12-12, p. 419)
Somatic motor neuron AP arrives (1st graph)
Skeletal muscle AP ( 2nd graph)
Muscle Contraction (3rd graph)
– Latent period
• Short delay between muscle AP and beginning
of muscle tension development
• Delay represents timing for the E-C coupling to
occur
– Twitch: a single contraction-relaxation cycle in
skeletal muscle
Figure 12-12
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Muscle Contraction-Relaxation Cycle
3rd graph
Once contraction begins, muscle tension increase
steadily up to a maximum value as crossbridge
interaction increases
Tension then decreases (relaxation phase)
During relaxation, elastic elements of the muscle return
the sarcomeres to their resting length
Figure 12-12
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Muscle Contraction-Relaxation Cycle
3rd graph
A single AP evokes a single twitch
The speed with which a muscle twitch develops
tension (rising slope of curve, bottom graph), the
maximum tension (height of the curve), and the twitch
duration (width of the curve) all vary, depending on the
muscle fiber
Figure 12-12
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Skeletal Muscle and ATP Requirements (p. 420-421)
The amount of ATP in a muscle fiber at any one time is
sufficient for approximately 8 muscle twitches
Phosphocreatine
– Backup energy source in muscle
– Has high-energy phosphate bonds
– These are made from creatine + ATP when muscle
is resting
Phosphocreatine
– When muscles are active, the high-energy P is
transferred to an ADP molecule to make more ATP
Creatine kinase (phosphokinase)
– The enzyme used to make the above transfer
– Muscle cells contain lots of this enzyme
– Elevated blood levels of this enzyme indicate
muscle damage (can differentiate between skeletal
and cardiac muscle damage)
Figure 12-13
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Carbohydrates, particularly glucose, are the most rapid
and efficient source of energy for ATP production
Glucose gets metabolized to pyruvate through
glycolysis
When adequate oxygen is present, then pyruvate is
further broken down in the citric acid cycle, yielding 30
ATP per molecule of glucose
During strenuous exercise, when oxygen levels fall in
the muscle tissue, glucose is metabolized to lactate
(via fermentation), and yields only 2 ATP per molecule
of glucose
Fatigue
– When muscle energy demands outpace the
available ATP
– Most studies show that, even intense exercise only
uses 30% of the muscle ATP
Fatigue (fig. 12-14, p. 421)
Central Fatigue
– Psychological effects
– Subjective feelings of tiredness, etc.
– May be a protective mechanism
• Psychological fatigue precedes muscle fatigue
Peripheral Fatigue
– May be at any level from neuromuscular junction
to muscle contractile elements
– Most studies indicate that this type of fatigue
usually has to do with excitation-contraction failure
(E-C coupling)
Figure 12-14
Copyright © 2010 Pearson Education, Inc.
Muscle Fiber Types
Slow-Twitch Oxidative (ST or type I)
– Red muscle (lots of myoglobin)
– Slowest speed
Fast-Twitch Oxidative-Glycolytic (FOG or type IIA)
– Red Muscle (lots of myoglobin)
– Intermediate speed
Fast-Twitch Glycolytic (FG or type IIB)
White Muscle (not much myoglobin)
Fastest speed
Muscle Fiber Types
Myoglobin: red O2 binding pigment
– The more myoglobin that is present, the faster O2
can get transported into the muscle fiber
Human muscles are a mixture of fiber types
Ratio of the types in an individual muscle will vary from
muscle to muscle and also from one individual to
another
Muscle Fiber Types: speed
Fast-twitch muscle fibers (type II) develop tension 2-3
times faster than the slow-twitch fibers
The speed is determined by the isoform of myosin
ATPase present in the thick filament
Fast-twitch fibers can split ATP very rapidly and
therefore can complete multiple contractile cycles more
rapidly than slow-twitch fibers
Muscle Fiber Types: Duration
Duration of contraction
– Varies according to fiber type
– Determined by how fast Ca++ is removed from the
cytosol
– Sarcoplasmic reticulum removes the Ca++
Fast-twitch fibers
– Pump Ca++ back into the sarcoplasmic reticulum
faster than slow-twitch fibers do
Muscle Fiber Types: Duration
Fast-twitch fibers
– Twitches last approximately 7.5 msec
– Useful for fine, quick movements
– Playing the piano, etc.
Slow-twitch fibers
– Twitches may last up to 10 times longer than slowtwitch
– These muscles are used almost constantly for
maintaining posture, standing, walking, etc.
Muscle Fiber Types: Fatigue Resistance
Glycolytic fibers (fast-twitch type IIB)
– Rely primarily on anaerobic glycolysis
(fermentation) for ATP
– H+ accumulation from this contributes to acidosis
(extracellular pH less than 7.38)
– Acidosis contributes to development of fatigue
– Glycolytic fibers fatigue more easily than oxidative
fibers
Oxidative fibers (slow-twitch, fast-twitch type IIB)
– Rely primarily on oxidative phosphorylation for ATP
– Have more mitochondria than glycolytic fibers do
– Have more blood vessels than glycolytic fibers
also
– Have lots more myoglobin (helps oxygen diffuse
into the muscle fiber)
– Have a smaller fiber diameter
Oxidative fibers (slow-twitch, fast-twitch type IIB)
compared to the glycolytic fibers
– Oxidative fibers maintain a better supply of O2 than
the other type because of
• Smaller diameter, shorter distance for O2 to
diffuse
• Presence of more myoglobin molecules which
can then transport more O2
• More capillaries to bring in O2 and nutrients
Glycolytic fibers compared to Oxidative fibers
– White muscle, due to lots less myoglobin
– Muscle fibers of glycolytic type are larger in
diameter
– Fewer blood vessels present in the glycolytic fibers
– Glycolytic fibers are more likely to run out of O2
after repeated contractions
– Also, they fatigue more rapidly than the other type
Fast-twitch oxidative-glycolytic fibers
These have properties of both oxidative and glycolytic
fibers:
– Smaller than fast-twitch glycolytic fibers
– Use both oxidative and glycolytic metabolism to
produce ATP
– More fatigue-resistant than the fast-twitch
glycolytic fibers
Figure 12-15
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Table 12-2
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Muscle Tension and Sarcomere Length
In a muscle fiber, the tension developed during a twitch
is a direct reflection of the length of the individual
sarcomere before contraction
Sarcomere optimum length: neither too short or too
long
Normal resting length of skeletal muscles usually has
sarcomeres at their optimum length before a
contraction
Figure 12-8
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Muscle Tension and Sarcomere Length
At the molecular level, the tension a muscle fiber can
generate is directly proportional to the number of
crossbridges formed between the thick and thin
filaments
If the fibers start a contraction at a very long
sarcomere length (fig. 12-16e), then the thick and thin
filaments barely overlap and very few crossbridges can
form
The sliding filaments would only have minimal
interaction and could not generate much force
Figure 12-16
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Muscle Tension and Sarcomere Length
At optimum sarcomere length (fig. 12-16c), the
filaments would be able to form lots of crossbridges,
generating optimum force
If the sarcomere is shorter than optimum length (fig.
12-16b), there is too much overlap between thick and
thin filaments
The thick filaments can move the thin filaments only a
short distance before the thin filaments from opposite
ends of the sarcomere start to overlap
Muscle Tension and Sarcomere Length
If the sarcomere is so short that the thick filaments run
in to the Z-disks (fig. 12-16A), then myosin can't find
binding sites for crossbridge formation and muscle
tension would decrease very rapidly
Summary:
Development of single-twitch tension in a muscle fiber
is a passive property that depends on both filament
overlap and sarcomere length
Figure 12-16
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Summation of Muscle Twitches
The force generated by the contraction of a single
muscle fiber can be increased by increasing the rate
(frequency) at which muscle action potentials stimulate
the muscle fiber
Typical muscle action potential lasts 1-3 msec
A muscle contraction may last 100 msec
Tension: force created by a contracting muscle
Figure 12-17a
If repeated APs are separated by long time intervals,
the muscle fiber can relax completely between stimuli
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Figure 12-17b
If the time between APs is shortened, the muscle
fiber can't relax completely, resulting in a more
forceful contraction
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Tetanus
If action potentials continue to stimulate the muscle
fiber repeatedly at short intervals (high frequency),
relaxation between contractions diminishes until the
muscle fiber reaches a state of maximal contraction or
tetanus
Incomplete (unfused) tetanus
– Stimulation rate not maximal, fiber relaxes slightly
between stimuli
Complete (fused) tetanus
– No relaxation, reaches maximum tension and
remains there
Figure 12-17c
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Figure 12-17d
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Figure 12-17
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Motor Unit
Consists of one somatic motor neuron and the muscle
fibers it innervates
When the motor neuron sends an action potential, all
of the fibers in the motor unit contract
One somatic motor neuron innervates many muscle
fibers, but each individual fiber is only innervated by a
single neuron
Figure 12-18
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Motor Unit
Fine motor actions (eye, hand movements)
– Motor unit has few fibers (as few as 3-5)
– Muscle response can be quite small
– Allows for fine gradations of movement
Gross motor actions (standing, walking)
– Each motor unit can have hundreds to thousands
of fibers
– Gastrocnemius (calf muscle) has 2,000 fibers in
each motor unit
Motor Unit
Within each motor unit, all the muscle fibers are of the
same type
– Fast-twitch motor unit
– Slow-twitch motor unit
The neuron controls the type of muscle fiber that
develops
– During development, each somatic motor neuron
secretes a growth factor that directs the
differentiation of all the muscle fibers in its motor
unit so that they all become the same type
Inheritance and Muscle Type
Endurance athletes (distance runners, cross-country
skiers) have lots of slow-twitch fibers
Sprinters, hockey players, weightlifters have a lot of
fast-twitch fibers
Athletic training can modify muscle fiber composition
because muscle fiber types show some plasticity
Endurance training can enhance the aerobic capacity
of some fast-twitch muscle fibers until they are almost
as fatigue-resistant as slow-twitch fibers
Table 12-2
Copyright © 2010 Pearson Education, Inc.
Endurance Training (continued)
The conversion of fast-twitch to slow-twitch fibers
occurs only in the muscles being trained
Endurance training also increases the number of
mitochondria and capillaries, thus increasing the
aerobic capacity of the fibers even more
High altitude training increases aerobic capacity also
by increasing the number of RBCs in the blood
This allows the blood to carry more oxygen
Contraction Force and Recruitment
Muscles are composed of multiple motor units of
different types (fig. 12-18, p. 426)
This diversity allows the muscle to vary contraction by
– Changing the types of motor units that are active
– Changing the number of motor units that are
responding at any one time
Figure 12-18
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Contraction Force and Recruitment
Recruitment
– The force of contraction in a skeletal muscle can
be increased by recruiting additional motor units
– Recruitment is controlled by the nervous system
– Proceeds in a standardized manner
• A weak stimulus activates only neurons with the
lowest thresholds
• These control the slow-twitch fibers
–Fatigue-resistant
–Generate minimal force
As the stimulus increases in strength, additional motor
neurons, with higher thresholds begin to fire
– These stimulate motor units of fast-twitch
oxidative-glycolytic fibers
– Fatigue-resistant
– Since more motor units are now contracting, the
force being generated is greater than before
As the stimulus continues to increase, additional motor
neurons, with the highest thresholds, begin to fire
– These stimulate motor units of the glycolytic fasttwitch fibers
– At this point, the muscle contraction is approaching
its maximum force
Asynchronous Recruitment
The nervous system avoids muscle fatigue in
sustained contractions by asynchronous recruitment of
motor units
This means that the different motor units take turns
maintaining muscle tension
The alternation of active motor units allows some of
the motor units to rest between contractions, thus
preventing fatigue
Asynchronous Recruitment
This type of recruitment prevents fatigue only in submaximal contractions
In high-tension, sustained contractions, the individual
motor units may reach a state of unfused tetanus in
which the muscle fibers cycle between contraction and
partial relaxation
Usually don't notice this cycling since the different
motor units are contracting and relaxing at slightly
different times
Asynchronous Recruitment
The contracting and relaxing average out and seem to
be one smooth contraction
As different motor units start to fatigue, we become
unable to maintain the same amount of tension in the
muscle
As this happens, the force of the contraction gradually
decreases
Table 12-3
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Next:
Chapter 13
Integrative Physiology I:
Control of Body Movement
(in the syllabus as Chapter 13: Reflex and Motor
Control)